Phase-Matched Second-Harmonic Generation from Metasurfaces Inside Multipass Cells

\nWe demonstrate a scalable approach to increase conversion efficiencies of nonlinear metasurfaces by incorporating them into multipass cells and allowing the pump beam to pass several times through the metasurfaces. As a proof of principle, we achieve phase matching of the second-harmonic generation (SHG) signal with superlinear dependence on the number of passes. We measure an order of magnitude enhancement in the SHG signal when the incident pump traverses the metasurface up to 9 passes. The generic nature of our approach holds promise for diverse applications in nonlinear optics.\n